According to the guide on my walking tour of The Rocks neighborhood in Sydney, Australia, when the plague hit the city around 1900, a bounty was placed on rats to encourage people to kill them, since it was known that rats bore the fleas that communicated the disease to humans. The intent of the bounty was plain enough: reducing the number of rats to reduce the spread of the plague. An unintended consequence, however, was that residents, tempted by the rat bounty, began breeding rats.

The law of unintended consequences is often associated with Robert Merton, though its general spirit appears in various forms, not least in Adam Smith’s invisible hand notion, and is somehow delightful in its chaos, as if Nature continually thumbs her nose at our attempts to control her.

The idea is that when people intervene in systems with a lot of moving parts, especially ecologies and economies, because of the complex interrelationships among parts of the system, the intervention will have effects beyond those that were intended, including many that were unforeseen or unforeseeable.

Examples abound. Returning to Australia, one of the best known examples of an unintended consequence is the case of rabbits, brought by the First Fleet as food, released into the wild for hunting, with the unintended consequence that rabbit populations grew to staggering proportions, causing untold ecological devastation, in turn leading to the development of measures to control the rabbits, including an exceptionally long fence, which had the unintended consequence of guiding three young girls home in the 1930s, which in turn had the unintended consequence of inspiring an award-winning motion picture (Rabbit Proof Fence) with a soundtrack by Peter Gabriel.

These chains of consequences occur because making changes to one part of a system with many interacting parts leads to changes in other parts of the system. Because many of the systems that we try to influence are complex but incompletely understood—bodies, habitats, markets—there are bound to be consequences that are difficult to predict.

This is not to say that the consequences will always be undesirable. Recently, certain municipalities changed the laws governing the use of marijuana, making it easier to obtain for medical purposes. The law might or might not have reduced the suffering of glaucoma victims, but data from traffic accidents suggest that the change in the legislation did reduce fatalities on the road by about 9%. (People substituted marijuana for alcohol and apparently drive better stoned than drunk.) Saving drivers’ lives was not the intent of the law, but that was the effect.

Another example, smaller in scale though closer to my heart, was the recent abrupt increase of parking rates by a third in University City, Philadelphia, where I work. The intent of the law was to raise revenue to help fund the city’s schools. An unintended consequence—because students seem disinclined to pay the higher price—is that I can rely on getting a parking spot when I have to drive to school.

Intervention in any sufficiently complicated system is bound to produce unintended effects. We treat patients with antibiotics, and we select for resistant strains of pathogens. We artificially select for wrinkly-faced bulldogs, and less pleasant traits, such as respiratory problems, come along for the ride. We treat morning sickness with thalidomide, and babies with birth defects follow.

In the economic sphere, most policies have various sorts of knock on effects, with prohibitions and bans providing some of the most profound examples, including, of course, Prohibition in America, which spun off various consequences including, arguably, the rise of organized crime in the United States. Because governments typically only ban things for which people have a taste, when bans do arise, people find ways to satisfy these tastes, either through substitutes or black markets, both of which lead to varied consequences. Ban sodas, give a boost to sports drink sales. Ban the sale of kidneys, spawn an international black market for organs and underground surgeries. Ban hunting mountain lions, endanger local joggers. (Not because joggers are substituted for mountain lions as prey for hunters; the ban increased the mountain lion population, who in turn menaced joggers.)

There is something oddly beautiful about the tendrils of causality in complex systems, holding the same appeal that we find in the deliberate inelegance of Rube-Goldberg machines. And none of this is to say that the inevitable possibilities of being surprised by our interventions means that we must give in to pessimism. Rather, it is a reminder to have caution and humility. As we gradually increase our understanding of large, complicated systems, we will develop new ways to glimpse the unintended consequences of our actions. We already have some guiderails—people will substitute for banned or taxed products; removing one species in an ecology typically penalizes populations that prey on them and aids species that compete with them, and so on—so while there will probably be always unintended consequences, these consequences won’t always be completely unanticipated.